Effects of seasonal water-level fluctuation on soil pore structure in the Three Gorges Reservoir,China

Inundation of the Three Gorges Reservoir has created a 30-m water-level fluctuation zone with seasonal hydrological alternations of submergence and exposure, which may greatly affect soil properties and bank stability. The aim of this study was to investigate the response of soil pore structure to seasonal water-level fluctuation in the reservoir, and particularly, the hydrological change of wetting and drying cycles. Soil pore structure was visualized with industrial X-ray computed tomography and digital image analysis techniques. The results showed that soil total porosity(? 100 ?m), total pore number, total throat number, and mean throat surface area increased significantly under wetting and drying cycles. Soil porosity, pore number and throat numberwithin each size class increased in the course of wetting and drying cycles. The coordination number, degree of anisotropy and fractal dimension were indicating an increase. In contrast, the mean shape factor, pore-throat ratio, and Euler-Poincaré number decreased due to wetting and drying cycles. These illustrated that the wetting and drying cycles made soil pore structure become more porous, continuous, heterogeneous and complex. It can thus be deduced that the water-level fluctuation would modify soil porosity, pore size distribution, and pore morphology in the Three Gorges Reservoir, which may have profound implications for soil processes, soil functions, and bank stability.

Three-dimensional visualization of soil pore structure using computed tomography

The geometric and spatial characteristics of pore structures determine the permeability and water retention of soils, which have important effects on soil functional diversity and ecological restoration. Until recently, there have not been tools and methods to visually and quantitatively describe the characteristics of soil pores. To solve this problem, this research reconstructs the geometry and spatial distribution of soil pores by the marching cubes method, texture mapping method and the ray casting method widely used in literature. The objectives were to explore an optimal method for three-dimensional visualization of soil pore structure by comparing the robustness of the three methods on soil CT images with single pore structure and porosity ranging from low (2–5%) to high (12–18%), and to evaluate the reconstruction performance of the three methods with different geometric features. The results demonstrate that there are aliases (jagged edges) and deficiency at the boundaries of the model reconstructed by the marching cubes method and pore volumes are smaller than the ground truth, whereas the results of the texture mapping method lack the details of pore structures. For all the soil images, the ray casting method is preferable since it better preserves the pore characteristics of the ground truth. Furthermore, the ray casting method produced the best soil pore model with higher rendering speed and lower memory consumption. Therefore, the ray casting method provides a more advanced method for visualization of pore structures and provides an optional technique for the study of the transport of moisture and the exchange of air in soil.

Soil pore identification with the adaptive fuzzy C-means method based on computed tomography images

The complex geometry and topology of soil is widely recognised as the key driver in many ecological processes. X-ray computed tomography (CT) provides insight into the internal structure of soil pores automatically and accurately. Until recently, there have not been methods to identify soil pore structures. This has restricted the development of soil science, particularly regarding pore geometry and spatial distribution. Through the adoption of the fuzzy clustering theory and the establishment of pore identification rules, a novel pore identification method is described to extract pore structures from CT soil images. The robustness of the adaptive fuzzy C-means method (AFCM), the adaptive threshold method, and Image-Pro Plus tools were compared on soil specimens under different conditions, such as frozen, saturated, and dry situations. The results demonstrate that the AFCM method is suitable for identifying pore clusters, especially tiny pores, under various soil conditions. The method would provide an optional technique for the study of soil micromorphology.

Quantitative analysis of soil pores under natural vegetation successions on the Loess Plateau

Soil pore is a key attribute of the soil structure that affects soil reservoir under natural vegetation recovery on the Loess Plateau.This study is to quantitatively analyze soil pore parameters,measured with Computed Tomography(CT) at 15-57 mm depths under five different vegetation succession stages using a concept of substituting "space" for "time" in the Ziwuling Forest Region of the Loess Plateau.The results showed that the soil pore parameters,such as pore number,porosity,circularity,and fractal dimension,increased significantly under the natural vegetation successions and varied with the pattern climax community stage】pioneering arbor community stage】scrub community stage】herbaceous community stage】abandoned farmland stage,indicating that natural vegetation recovery could remarkably improve soil pore characteristics.With the vegetation succession,this positive effect will be strengthened gradually,which means that the soil pore parameters may appear to be best at the climax community stage.Soil organic matter content increased linearly with the soil pore parameters(P【0.001).Increased organic matter accumulation was one of the major reasons for the changes in soil pore characteristics in natural vegetation succession.

Wave-induced pore water pressure in marine cohesive soils

Cyclic triaxial tests and numerical analyses were undertaken, in order to evaluate the wave-induced pore water pressure in seabed sediments in the Hangzhou Bay. The cyclic triaxial tests indicate that the rate of pore water pressure generation in cohesive soils decreases with time, and the development of the pore water pressure can be represented by a hyperbolic curve. Numerical analyses, taking into account the generation and dissipation of pore water pressure simultaneously, suggest that the pore water pressure buildup in cohesive soils may increase with time continuously until the pore water pressure ratio approaches to 1, or it may decrease after a certain time, which is controlled by drain conditions. These phenomena are different from those in sands. For waves with a retum period of 100 a in the Hangzhou Bay, if the wave duration is more than 60 h, then the pore water pressure ratio will be close to 1 and soil fabric failure will take place.

Soil pressure and pore pressure for seismic design of tunnels revisited: considering water-saturated, poroelastic half-space

This paper describes a systematic study on the fundamental features of seismic soil pressure on underground tunnels, in terms of its magnitude and distribution, and further identifi es the dominant factors that signifi cantly infl uence the seismic soil pressure. A tunnel embedded in water-saturated poroelastic half-space is considered, with a large variety of model and excitation parameters. The primary features of both the total soil pressure and the pore pressure are investigated. Taking a circular tunnel as an example, the results are presented using a fi nite element-indirect boundary element(FE-IBE) method, which can account for dynamic soil-tunnel interaction and solid frame-pore water coupling. The effects of tunnel stiffness, tunnel buried depth and input motions on the seismic soil pressure and pore pressure are also examined. It is shown that the most crucial factors that dominate the magnitude and distribution of the soil pressure are the tunnel stiffness and dynamic soil-tunnel interaction. Moreover, the solid frame-pore water coupling has a prominent infl uence on the magnitude of the pore pressure. The fi ndings are benefi cial to obtain insight into the seismic soil pressure on underground tunnels, thus facilitating more accurate estimation of the seismic soil pressure.

黄河三角洲潮滩表层沉积物在潮波和波浪作用下的孔压响应及固结机理现场试验研究

为了研究黄河三角洲潮滩表层粉质土在波浪和潮波作用下的孔压响应及对沉积物固结状态影响,在黄河三角洲潮滩上现场取土在试坑内配置了模拟黄河口快速沉积形成的流体状堆积物,利用静力触探试验、孔隙水压力测试手段,对比研究了试坑扰动恢复土体和潮滩原状土体在波浪和潮波作用下的孔压响应,并探讨了黄河三角洲潮滩表层粉质土非均匀固结机理。研究发现:在潮波作用下,潮滩和试坑土体产生了不同的孔压响应,在涨退潮过程中试坑土体超静孔隙水压力经历了负-零-正的变化,相当于经历了一次加卸荷过程,而潮滩土体仅产生了负值超静孔隙水压力。在潮波与波浪相互耦合作用下,潮滩浅部土体产生了明显的孔隙水压力积累,在深度0.3 m附近出现超静孔隙水压力最大值。潮滩表层土体强度沿深度从上到下呈现高-低-次高不均匀固结现象,产生这一现象的机理与潮滩土体在潮波、波浪和潮波相互耦合作用下孔压响应特点有关,特别是潮滩0.3 m深度附近土体受波浪影响显著,长期作用下形成硬壳层。

物理化学效应对膨胀土收缩特性的影响机制

膨胀土由于其骨架带有较多的固定负电荷,层间存在与负电荷平衡的可交换阳离子,使得土体呈现较强的胀缩性。研究结果表明,膨胀土的胀缩性会受到孔隙溶液化学成分的影响。选用广西地区的强膨胀土作为研究对象,开展了不同浓度的NaCl溶液对膨胀土土-水特征曲线和收缩曲线影响的试验研究,引入了粒间应力的概念对收缩曲线进行描述,该粒间应力考虑了渗透、毛细和吸附的影响。结果表明:孔隙盐溶液是通过渗透吸力对土-水特征曲线产生影响,对基质吸力的影响较小。土样在脱湿过程中的收缩变形是由粒间应力来控制的,类似于加压固结现象。大部分的收缩都发生在毛细阶段,为弹塑性变形;吸附阶段的收缩较少,为弹性变形。通过识别压缩曲线上的弹塑性分界点可以得出毛细和吸附作用的分界点,该分界点与独立测量的不同密实度下的持水曲线结果一致。结果表明,粒间应力能够更好地描述膨胀土的化学-力学行为,特别是在低含水率条件下。

改进UNet-VAE网络的土壤多类型孔隙三维分割方法

不同类型土壤孔隙结构会随生物活动和非生物作用发生形变,从而影响其生态功能,准确分割不同类型的孔隙结构对于研究孔隙结构与生态功能演变关系具有重要意义。针对单个类别孔隙的分割方法分割精度低、分类标准单一、鲁棒性差,无法准确分割和判别生物孔隙、裂隙等相交部分孔隙结构的问题。该研究针对不同类型孔隙尺度差距大的特点,提出了一种改进UNet-VAE网络模型,实现土壤多类型孔隙分割。改进UNet-VAE网络引入多尺度特征融合注意力模块,以实现多尺度信息融合和冗余信息筛选。结合变分自动编码器生成网络(variational autoencoder,VAE),引入噪声和辅助损失函数,以增强网络的泛化能力和鲁棒性。试验结果表明:改进UNet-VAE方法在土壤多类型孔隙(裂隙、生物孔、不规则孔隙和球状孔隙)三维分割中达到了93.83%的平均准确率,与次优VNet方法相比,平均准确率、精确率、召回率和F1值分别提升了3.32,5.06,8.97和8.63个百分点,特别是对于不规则孔隙4项指标分别提升了4.88,15.46,15.70和15.50个百分点,表明改进UNet-VAE法可准确分割多类型孔隙,也验证了深度学习技术在多类型孔隙判别的有效性,可为揭示土壤孔隙结构与演化研究提供有效工具。

接种蚯蚓对有机茶园土壤结构及有机碳库的影响

为探讨接种蚯蚓对有机茶园土壤团聚体、孔隙分布及有机碳库的影响,设置375条/m^(2),250条/m^(2),125条/m^(2)三种蚯蚓投放密度,将威廉腔环蚓接种于信阳有机茶园土壤中。结果表明:接种蚯蚓能显著提升>2mm大团聚体的比例而降低<0.25mm粒径的微团聚体的比例;提高水稳性小团聚体(0.25mm≤d≤2mm)比例,但对水稳性大团聚体d>2mm的影响不显著;高密度蚯蚓处理平均重量直径(MWD),平均几何直径(NWD)以及分型维数(D),团聚体破坏率(PAD)显著高于其他密度处理及对照(CK)。接种蚯蚓使得结构性孔隙(土壤颗粒间和团聚体间孔隙)所占比重增加,其中以团聚体间孔隙增加最为显著,其次为颗粒间孔隙,对由粘粒及矿物层间形成的毛管孔隙分布无显著影响;随着接种密度的增加,颗粒间孔隙度变化不显著而团聚体间孔隙度的相对值和绝对值均显著增加。添加蚯蚓能显著增加土壤活性有机库总量。接种密度的增加对活性有机碳库总量影响不大,但对土壤呼吸速率提升较多,故从温室气体排放角度考虑低接种密度更合适。可见,有机茶园中少量接种蚯蚓即能达到改良土壤结构的效果,并能缓解由于采茶造成的土壤压实问题。

有机物料还田对草甸土孔隙结构及玉米产量的影响

为了研究有机物料还田对草甸土孔隙结构及玉米产量的影响,优化草甸土耕作层结构、提高作物产量,2019年以吉林省公主岭市的草甸土为试验地,在玉米播种前设置了秸秆浅混还田(CT,0~15cm)、秸秆深混还田(STS,0~35cm)、秸秆和有机肥深混还田(STSM,0~35 cm)、无秸秆还田常规耕作(CK)4个处理。对玉米收获后的土壤进行取样分析,测定0~35 cm土层土壤物理性质和孔隙结构,并采用结构方程模式、Mantel test和随机森林方法分析土壤孔隙结构调控玉米产量的途径。结果表明,经CT、STS和STSM处理后的0~15 cm土层土壤容重与CK处理相比显著下降了5.2%~7.8%(P<0.05),而田间持水量和饱和导水率均呈现上升趋势,分别显著增加了7.8%~16.6%和77.5%~325%(P<0.05);STS和STSM处理进一步显著改变了>15~35 cm土层上述3个土壤物理性质。CT扫描结果显示,有机物料还田显著改善了相应土层土壤结构。与CK处理相比,CT、STS和STSM处理显著增加了0~15 cm土层>1000µm孔隙数量和孔隙度;与CT处理相比,STS和STSM处理>15~35 cm土层>1000µm孔隙数量和孔隙度分别显著增加了1.2倍~1.3倍和47.5%~58.9%(P<0.05)。有机物料还田增加了土壤孔隙的复杂性和连通性。与CK处理相比,CT、STS和STSM处理显著增加了0~15 cm土层的各向异性和分形维数,降低了欧拉数(P<0.05);STS和STSM处理显著改善了15~35 cm土层土壤孔隙结构。不同处理间玉米产量表现为STSM>STS>CT>CK。结构方程模型结果显示,0~15 cm土层土壤孔隙结构可以直接影响玉米产量或者通过影响容重和饱和导水率间接影响玉米产量,而15~35 cm土层土壤孔隙结构只能通过影响田间持水量间接影响玉米产量。0~35 cm土层土壤物理性质和孔隙结构能够解释玉米产量的81.9%,15~35 cm土层对玉米产量的贡献大于0~15 cm土层。有机物料深混还田通过改变0~35 cm土层草甸土物理性质和孔隙分布,增加孔隙结构的复杂性和连通性,优化耕层结构,具备提高玉米产量的潜力,其中秸秆和有机物配合施用的效果优于单独秸秆还田。如果草甸土分布区有机肥源充足,建议利用秸秆和有机肥料进行深混还田可建立良好的土壤肥力,可以有效增加玉米产量。

大荷载矿石堆场中深厚吹填软土的地基处理及后评价

吹填淤泥形成陆域是资源利用和环境保护方面的优质解决方案,但当前吹填软土形成陆域的工程中,多数为荷载较小的堆场、工业园区道路、绿化区等。以青岛港董家口港区矿石码头工程20万吨级散货泊位后方预留堆场区的地基处理工程为例,对该工程的设计、施工及使用后6年的情况进行详细阐述,从多个角度展示深厚吹填软土的演变情况,详细介绍土体含水量、孔隙比、抗剪强度等指标在不同时期的变化,对比施工期沉降计算和监测数据,分析不同时期抗剪强度指标下矿石堆存的高度。结果表明,在大荷载矿石堆场中采用吹填软土形成陆域是可行的,通过合理的地基处理方式和有序增加堆存荷载,最终可使矿石堆场达到满载堆存。

细粒土与粗粒土动力流化的孔压差异辨析

地基土体在动力作用下的液化对工程会产生灾害性影响。孔隙水压力是研究饱和土体转变为流体的重要表征,已有研究发现,细粒土在液化过程中即使孔压比(孔压/围压)未达到1,也已进入液化状态。本文将土颗粒概化为球体,考虑颗粒吸附水膜的影响,对由砂粒、粉粒、黏粒组成的饱和土体,在可发生流动的情况下,建立产生动力流化的理论模型,并分析了动三轴和振动台实验中孔压的变化。结合土体微结构特征,指出细粒土处于流化状态时,孔隙水压力因受结合水膜影响而导致孔压值减小,同时因颗粒形状(如板片等)的框架支撑作用也可减小孔压值。这一从土体微观组成的物理性质上进行的理论分析,有助于厘清以孔隙水压力判别细粒土与粗粒土发生动力流化标准不同的原因,有助于研究者深入理解土体液化的内在机制。

The Evolution of Pore Water Pressure in a Saturated Soil Layer between Two Draining Zones by Analytical and Numerical Methods

The building of the infrastructure on the compressible and saturated soils presents sometimes major difficulties. The infrastructure undergoes strong settlement that can be due to several phenomena of consolidation of the soils. The latter results from the dissipation of the excess pore pressure and deformation of the solid skeleton. Terzaghi theory led to the equation modeling the dissipation of excess pore pressure. The objective of this study is to establish solutions, by analytical and numerical method, of the equation of the pore water pressure. We considered a compressible saturated soil layer, between two drainage areas and subjected to a uniform load. Separation of variables is used to obtain an analytical solution and the finite element method for the numerical solution. The results obtained by the finite element method have validated those of analytical resolution.

基于能量的振冲碎石桩加密效果评价方法

采用常规检测手段进行处理地基的工后质检存在时间滞后与空间局部的问题,不能及时反映处理地基的整体加固效果。提出了一种基于能量的碎石桩复合地基桩周土振动加密效果评价方法,为实现桩周土加密效果的准实时评价提供参考。首先,根据地基弹性波传播理论建立振冲施工过程中桩周土吸收波动能量的计算方法,通过不排水动三轴试验建立典型饱和粉砂土基于吸收能量的超静孔压模型,为进一步根据固结理论预测处理后复合地基桩周土密实度提供依据;其次,依托某工程水电站深厚坝基碎石桩处理工程实例,采用桩周土超静孔压峰值和处理后孔隙比两个关键物理量对方法进行了检验,发现与施工现场实测超静孔压和工后质检得到的孔隙比相比,方法的预测精度达到90%左右,说明基于能量的振冲碎石桩加密效果评价方法具有可行性。

排渗失效下尾矿库溃坝三维模型试验研究

针对尾矿库渗透破坏模式,为克服二维试验槽边壁效应对模型渗流走向及溃决位置的影响,本文以某尾矿库为原型,基于自主设计的三维尾矿库溃坝模型试验平台,进行了排渗设施失效诱发的尾矿库溃坝模型试验研究,揭示了该尾矿库的溃决模式及溃决机理。结果表明:(1)排渗失效导致的溃坝呈现局部流土破坏-坝体累计变形-深层滑动面发展贯通-整体突发失稳的渐进伴突发型溃决演化模式;(2)坝体孔隙水压力骤变是坝体发生滑动破坏的关键响应指标,预示着坝体将在短时间内由滑动体前后缘局部散浸软化、流土、塌陷转变为大范围溃决,可作为坝体溃决事故的前兆信息;(3)排渗失效导致的尾矿库溃坝过程具有渐进性和突发性的特点,发生散浸与流土破坏的过程时间较长,发生深层滑动和溃决的时间短暂,该特征可为尾矿库渗透破坏隐患治理及坝体溃决应急响应提供参考。研究结果对尾矿库溃坝的灾害预防具有重要的现实意义。

冲击荷载下含夹层饱和砂土孔压变化规律分析

砂土中夹层的性状会影响饱和砂土孔压发展,从而影响砂土层变形。为研究夹层位置、厚度和种类等不同夹层状态下砂土液化过程中的孔压变化规律,设计了冲击荷载作用下层状砂液化试验,建立了含夹层饱和砂土理论模型,并将试验结果与理论分析进行对比。结果表明:含夹层饱和砂土的孔压发展呈现3个阶段,即快速上升、快速消散、缓慢消散阶段。高渗透性夹层高度越高,其下方土层孔压快速消散时长越短,越快趋于稳定值,但消散总时长无明显影响;低渗透性夹层高度或厚度的增大,均会使夹层上方孔压快速消散阶段速率加快,孔压消散稳定阶段延长,孔压消散总时长随之线性增长;同时,孔隙水会在低渗透性夹层下方形成水膜,夹层高度或厚度的增加均会使水膜持续时间增长,但水膜形态主要受夹层厚度影响。试验结果与理论分析较为一致,说明了试验的可靠性。

超硫酸盐渍土固化强度及冻融特性试验研究

地基强度不足和盐–冻胀破坏是导致盐渍土地区渠道衬砌结构发生破坏的主要原因。为解决盐渍土强度不足的问题,降低盐渍土对环境温度的敏感性,首先,开展联合固化剂对超硫酸盐渍土强度的影响研究,并结合微观测试结果分析,构建联合固化剂掺量与强度之间的关系,对固化效果作出评价;其次,借助压汞试验对微观孔结构进行分析;最后,通过冻融循环试验揭示盐–冻胀力及盐–冻胀量的变化规律。结果表明:固化盐渍土强度增长效果显著,水化产物增强了盐渍土颗粒间的胶结作用。当联合固化剂掺量从0增加至30%时,固化盐渍土的孔隙率逐渐降低,孔径从超大孔向中小微孔演变;模型计算得到的分形维数逐渐增大,其中,Menger海绵模型与Neimark模型表现为多尺度分形,在中孔隙(0.1μm≤孔径<1.0μm)范围分形特征不明显,由热力学模型计算得到的分形维数DT在合理区间2.824~2.849变化,很好地表征了孔隙表面的分形特征。冻融循环10次后,相较未固化盐渍土,固化盐渍土盐–冻胀量削减了117%;盐渍土盐–冻胀力波动范围随冻融次数的增加而逐渐增加,固化盐渍土盐–冻胀力转变为周期性衰减,掺入的联合固化剂有效弱化了盐渍土盐–冻胀力。结合工程实际,建议渠道地基加固中水泥∶粉煤灰∶硅灰∶脱硫石膏质量比为1.00∶1.67∶0.83∶1.00,联合固化剂掺量为30%。

施肥对热带砖红壤胶林土壤水分特征曲线的影响及模型优选

[目的]探明热带橡胶林施肥措施对土壤持水能力及孔隙特征的影响,并优选描述砖红壤胶林土壤水分特征曲线的模拟模型。[方法]采用沙箱排水法结合压力膜仪法测定砖红壤橡胶林5种施肥处理(有机肥混施化肥(H)、施化肥(C)、种植绿肥(G)、施专用肥(S)及不施肥(CK))的全吸力段土壤水分特征曲线,应用Brooks-Corey(BC)、van Genuchten(VG)、Gardner和Log-Normal Distribution(LND)4种经验模型拟合,分析了不同模型的适用性及拟合精度。[结果]0—5 cm土层,在低吸力段(pF≤3.01)C处理降幅最大(0.44 cm^(3)/cm^(3));在高吸力阶段(3.01<pF≤4.18)的降幅整体偏小,H处理的降幅最大(0.05 cm^(3)/cm^(3))。5—10 cm土层,在低吸力段C处理的下降幅度最大(0.52 cm^(3)/cm^(3)),高吸力段C处理的降幅最大(0.03 cm^(3)/cm^(3))。在0—5 cm土层中,CK处理的土壤有效含水率最大(0.19 cm^(3)/cm^(3))。在5—10 cm土层中,C处理的土壤有效含水率最大(0.11 cm^(3)/cm^(3))。0—5 cm土层,CK处理的土壤空隙占比(5.2%)和大孔隙占比(0.8%)最小,中等孔隙占比(48.7%)最高。5—10 cm土层中,C处理的土壤空隙和大孔隙的占比均最小。BC,VG,Gardner和LND 4种模型均可用来拟合砖红壤胶林不同施肥处理的土壤水分特征曲线(R 2≥0.84),不同模型的拟合精度由大到小依次为:BC,VG,LND和Gardner模型。[结论]0—5 cm土层不同施肥处理中CK处理是最优施肥管理方式,5—10 cm土层所有施肥处理中C处理是最优施肥管理方式。同时,BC模型是拟合研究区内不同施肥处理土壤水分特征曲线是最优模型。

统一边界条件下非饱和土一维固结理论研究

非饱和土固结理论中的边界条件对土体的固结有着重要的影响,它反映了土体在固结过程中边界的透气透水状态。针对实际工程中边界条件的复杂多样性,该文引入一个统一形式边界条件,构建了一个在瞬时均布荷载作用下,由顶面统一边界、底面完全不渗透边界组成的非饱和土一维固结计算模型。基于Fredlund非饱和土一维固结理论,采用Laplace变换等方法得到了Laplace域内超孔隙压力的解,再通过Laplace逆变换得到了时间域内相应解答。运用一个典型算例,设置合理的边界参数,将统一边界条件下的半解析解退化到几种常规边界条件下的解与文献中已有的结果进行比较,验证了所得解的正确性,并对非饱和土一维固结特性进行了简要分析。研究结果表明:统一边界条件下的半解析解相当于一个通解,具有广泛的适用性;通过改变相关边界参数的取值,可以模拟土层边界条件由完全不渗透到完全渗透的变化过程;固结过程中,边界条件对超孔隙气压力和超孔隙水压力的消散影响很大。因此,该文的研究具有一定的学术价值,所得的半解析解对不同边界条件下的实际工程具有参考价值。